The present invention relates generally to circulatory assist devices for the heart, and more particularly, to a pump having a high-speed, motor-driven actuator.
Circulatory assist procedures such as intra-aortic balloon counterpulsation increase coronary profusion and decrease myocardial oxygen consumption. During the normal cardiac cycle, the atrium first contracts (atrial systole) and the ventricles fill with blood through the atrioventricular valves. The ventricles then contract (ventricular systole) in response to an electrical impulse. Immediately after ventricular contraction starts, ventricular pressure rises abruptly causing the atrioventricular valves to close. The volume of blood within the ventricles just prior to the closure of the atrioventricular valves is known as ventricular preload. As the ventricles continue to contract, blood pressure in the ventricles continues to rise. These are known as isovolumetric contractions since muscular contraction occurs without a change in volume, and This portion of the cardial cycle results in a majority of myocardial oxygen consumption. The resistance in the arterial circulation against which the ventricle muscles must pump and the resultant left ventricular wall tension during systole is called afterload. When the pressure in the ventricles exceeds the pressure in the circulation, blood flows from the left ventricle into the aorta via the aortic valve, and from the right ventricle into the pulmonary artery via the pulmonic valve. At the termination of ventricular ejection, the ventricles relax. When the blood pressure within the ventricles drops below the systemic pressure, the aortic and pulmonic valves close signifying the onset of isovolumetric relaxation (ventricular diastole). When the ventricular pressure drops below the pressure in the atria, the atrioventricular valves open and the cycle starts again as the ventricles fill with blood.
Myocardial oxygen consumption is dependent upon heart rate, afterload, preload and contractility. When the myocardium is injured, the patient's heart action is insufficient to meet myocardial demand. With cardiac failure, the myocardial oxygen supply is reduced. The circulatory system compensates by increasing preload, afterload and heart rate. This in turn further increases myocardial demand and a deteriorating cycle develops where myocardial oxygen supply continues to decrease while demand continues to increase.
The intra-aortic balloon is used to assist the weakened heart. The balloon catheter is percutaneously or surgically inserted into the patient's aorta to inflate and deflate in conjunction with the cardiac cycle. In this connection, it helps circulation by increasing aortic pressure during diastole to augment coronary profusion, and it decreases aortic pressure during systole to lower muscular demands on the left ventricle.
During diastole, when the left ventricle is relaxed and the coronary arteries are filling with oxygenated blood, the balloon is timed to inflate. This increases the coronary circulation and is known as diastolic augmentation or increasing the diastolic pressure.
When the left ventricle contracts to pump blood into the aorta, the balloon is deflated to decrease the pressure against which the left ventricle has to function, thereby reducing the afterload, and aortic end--diastolic pressure and systolic pressure. Through a combination of diastolic augmentation and a reduction in aortic end--diastolic pressure, afterload decreases, cardiac output increases, and blood circulation through the coronary vessels increases.
There are many circulatory assist devices known in the art. However, most are dependent upon a complex arrangement of compressors and vacuum pumps which operate in conjunction with pressure and vacuum accumulators to inflate and deflate the balloon. Examples are disclosed in U.S. Pat. Nos. 3,769,960 to Robinson, 4,794,910 to Mushika, 4,832,005 to Takamiya et al., 5,158,529 to Kanai and 5,169,379 to Freed et al.
To inflate the balloon, compressed gas stored in a pressure accumulator is typically applied to one side (the drive side) of a diaphragm or safety chamber causing it to displace gas (e.g., helium) on the other side (the balloon side). The displaced helium enters the balloon and causes it to inflate. To deflate the balloon, a vacuum is then applied to the drive side. The diaphragm separates high-pressure drive gas from the relatively low balloon pressure (usually less than 50 millimeters of Hg). Most pumps limit the drive gas to 300 millimeters Hg for safety reasons so that in the event of a diaphragm rupture, the balloon will not be exposed to pressures any higher than the drive pressure. This limitation on drive pressure limits the speed with which balloons can be inflated.
The present invention is directed to overcoming the disadvantages inherent in the prior art by providing a balloon pump having a motor-driven diaphragm assembly in lieu of the standard pneumatic arrangement. Mechanical diaphragm displacement reduces the number and size of the individual drive components, eliminates high pressure drive gas and required systems for evacuating the same in the event of a diaphragm rupture, and permits better system response. A pump in accordance with the present invention is equally suited for use in a ventricular assist device and the background and detailed description with regard to intra-aortic balloon catheters is not meant to be limiting.